It’s mostly about consent. We can debate when and where sentience begins, but it begins somewhere and vegans hold a moral philosophy that says using another sentient being’s work product or body without their consent is immoral.

Note that I am not vegan myself but understand, if not agree with, their moral position.

And as another reply said, most vegans recognize it as a “best effort” philosophy, as they appreciate the impracticality of an absolutist stance. They are focused on “harm reduction”.